tec: How can you know there is no tooth-ferry with certainty? how can you know the earth is round with certainty?.
its in that sence dawkins use the word (as do all scientists).
Most atheists know there is no God, in the same way that most theists know there is a God.
Who? What do you base that on? Dawkins is certainly not amongst them, nor am I. There may be a god just as there may be a tooth ferry; only mathematics deal with certainty.